The heart maintains blood flow to the tissues of the body by its pumping action. Blood courses outward to all parts of the body through an arterial tree. The arterial tree has as its trunk the aorta, which receives blood as it is expelled from the heart. Major branches include the carotid arteries leading to the head and the brachial artery leading to each arm. Numerous smaller arteries and capillaries reach essentially all parts of the body. Blood pressure is pressure in the arterial system, i.e. pressure in the distribution part of the circulatory system.
Blood pressure varies with the beating of the heart. It peaks during systole, the contraction of the heart's left ventricle, during which blood is ejected into the aorta. The peak pressure is systolic pressure. As the left ventricle relaxes during diastole, blood pressure gradually falls to a minimum. The minimum pressure is the diastolic pressure. The ejection of blood into the aorta gives rise to the pulse felt in arteries.
Medical diagnostics relies heavily on measurement of blood pressure as an indication of cardiovascular function, cardiovascular morbidity and mortality. Absolute accuracy in measurement requires invasive techniques, i.e. the insertion into an artery of a needle or catheter. However, indirect and "noninvasive" measurement techniques are typically used. Lower cost and greater convenience and comfort for the patient dictate indirect measurement. The term "indirect" means that the measurement is not of actual pressure. A technique is "noninvasive" in that the body is not actually penetrated.
One method of indirect blood pressure measurement is the occluding-cuff auscultatory technique. An occluding-cuff is an unyielding cuff which is positioned around the arm of a patient for the test. Properly positioned and closed, the cuff positions and retains an air bladder over the brachial artery. The bladder occludes the brachial artery upon sufficient inflation. Auscultation is the act of listening. In medicine the term auscultation refers to the diagnostic monitoring of sounds made by internal body parts or organs. An example of auscultation is the use of a stethoscope to listen to heart or lung action. In the "occluding-cuff auscultatory technique" sounds are produced in the brachial artery by the intermittent flow of blood. Blood flow is intermittent as a result of pressure applied to the artery by the bladder. The diagnostically significant sounds are called Korotkoff sounds. The various Korotkoff sounds signify certain things to a trained user. The user takes pressure readings of bladder pressure as the various types of Korotkoff sounds are heard.
Korotkoff sounds associated with the beginning of intermittent blood flow through an artery and occurring just before the disappearance of all sound are the most significant. Phase I sounds occur at the highest pressure, where blood flow can just occur at the highest blood pressure. Phase V sounds occur at the lowest pressures at which blood flow can be interrupted at all. The measurements are commonly taken on the brachial artery to standardize readings. The first pressure is peak arterial pressure, i.e. systolic pressure. The lowest pressures are minimum arterial pressure, i.e. diastolic pressure.
Medical practitioners employ two familiar instruments in practicing this technique. One instrument is the common stethoscope, which is used to amplify the Korotkoff sounds. The second instrument is called a sphygmomanometer. The sphygmomanometer includes the cuff and bladder described above, a bulb for pressurizing the bladder with air, a valve for releasing air from the bladder and some type of pressure indicator. Higher quality sphygmomanometers have a mercury containing column as a pressure indicator. The column is graduated to indicate pressure in the bladder.
In practicing the technique, the medical practitioner applies the cuff around the upper arm of a patient and inflates the bladder to occlude the brachial artery. She then places the sound gathering end of the stethoscope over the brachial artery below the cuff. She releases air from the bladder by a manually actuated valve and notes the pressures at which valid Korotkoff sounds appear and again disappear.
Researchers have focused much effort relating to blood pressure measurement techniques on automating the detection of Korotkoff sounds. For example, U.S. Pat. No. 4,592,365 relates to an "Electronic Sphygmomanometer". The patent discloses a device for the automatic detection, verification and evaluation of Korotkoff sounds. Waveform analysis provides the basis for achieving these functions. Transducers provide cuff flex sensing relating to pressure waves, upstream and downstream microphones for detecting Korotkoff sounds, and a pressure transducer measuring the pressure in the sphygmomanometer. Korotkoff sound signals and blood pressure pulses, which are precursors to true Korotkoff sounds, are obtained from the separate transducers. Separate channels are provided for the sound and pressure wave data. Separate waveform analysis of the two signals can then be provided. Korotkoff sounds are verified dependent upon signal slope characteristics and proper phase relationship to the pressure pulse waveform. Upon Korotkoff sound verification, the pressure transducer reading is automatically noted.
Implicit in the focus on automation of the steps of blood pressure measurement is lack of confidence in the quality of human observation. There is some basis for such lack of confidence. In a front page article reported in the Saint Paul Pioneer Press on 9 Jan. 1991, studies have indicated wide spread failure to conform blood pressure measurement procedures with those recommended by the American Heart Association. The recommendations appear in the pamphlet "Recommendations for Human Blood Pressure Determination by Sphygmomanometers", published by the American Heart Association, 7320 Greenville Avenue, Dallas, Tex. 75231. For example, only about 20% of the practitioners take the recommended two readings and average those readings. Of those practitioners who take two readings, fewer still wait the recommended 30 seconds between readings. Some practitioners exhibit what is known as terminal digit preference. While these and other factors suggest unreliability in human observation, these causes do not necessarily mean that the human is not the best arbiter of the subtle variations which characterize Korotkoff sounds.
The causes of the poor performance by many practitioners included inconsistent or incomplete training and laziness. The occluding-cuff auscultatory technique is complex in terms of hand, eye and ear coordination. The practitioner must watch changing pressure indications in a column of mercury, listen for Korotkoff sounds and control the rate of deflation of the bladder. The user watches the pressure indications both to control the rate of deflation and to note the pressures at which the appropriate Korotkoff sounds occur. Both systematic and random error can result from this arrangement. Error may be induced by the number of simultaneous tasks the practitioner must do.